[chore] add standalone TA e2e test

[odubajDT]

Part-of: #4918

Motivation

The Target Allocator (TA) can run as a standalone component without the OpenTelemetry Operator — users deploy it directly with raw Kubernetes manifests when they need Prometheus target sharding across collectors without adopting the full operator lifecycle. Today, no e2e tests validate the TA in this standalone mode. All existing TA tests go through the operator-managed flow, leaving a gap in coverage for:

• Target distribution correctness across collectors
• Target redistribution on collector scale-up/down
• Standalone deployment viability

What this PR adds

A new Chainsaw e2e test suite (tests/e2e-ta-standalone/) that deploys the TA as a raw Deployment with real collector pods — no operator involved. The test validates the core TA functionality end-to-end.

Test flow

1. Deploy — RBAC, TA config, TA deployment, 2 collector replicas
2. Verify initial allocation — All 6 targets distributed across collector-0 and collector-1 via GET /jobs/test-targets/targets
3. Scale up to 3 — kubectl scale statefulset/collector --replicas=3
4. Verify redistribution — collector-2 receives targets, total remains 6
5. Scale down to 2 — Remove collector-2
6. Verify reassignment — collector-2's targets move back to remaining collectors

[github-actions]

E2E Test Results

 34 files  249 suites   2h 11m 16s ⏱️
 96 tests  96 ✅ 0 💤 0 ❌
253 runs  253 ✅ 0 💤 0 ❌

Results for commit fc2a6e1.

♻️ This comment has been updated with latest results.